A speaker module comprises a module shell and a speaker unit mounted in the module shell. The most important structure of a moving coil structure as the speaker unit comprises a vibration diaphragm, a voice coil fixedly connected on the vibration diaphragm, and a magnetic circuit system. The voice coil is located in a magnetic gap formed in the magnetic circuit system. The speaker unit of such a structure drives the vibration diaphragm to vibrate under the action of the magnetic circuit system when the voice coil receives an audio signal, and the ambient air is stirred for sound production, so as to realize the conversion of electric energy to sound energy.
Such a speaker unit will limit the application of the maximum power of a product when working under low frequency conditions. The main problem is that under high power, the voice coil working under the low frequency conditions will generate an excessive displacement. The excessive displacement causes a sharp increase in distortion, and a noticeable collision between the voice coil and the magnetic circuit system is even caused, resulting in irreversible damage to the speaker.
For the above problem, the current solution is to use an intelligent power amplifier control unit to control the power fed to the speaker unit, so as to reduce the power when the vibration displacement of the vibration system exceeds a predetermined level, but it is required to know the vibration displacement of the vibration system.
In the prior art, a solution for detecting the vibration displacement is that the voice coil and an external circuit are used as a sensor, to realize the monitoring of the vibration displacement of the vibration system by the real-time measurement of a speaker model and the real-time monitoring of an input signal. The premise of such solution is the assumption that the speaker has a theoretical model, for example, a vibration diaphragm stiffness coefficient Kms, a vibration system mass Mms, a force electric drive factor Bl, a damping factor Rms, a DC impedance Re, an inductance Le, etc. However, there is still a certain difference between the theoretical speaker model and an actual product, which results in a limited monitoring accuracy of the displacement. The low-frequency performances of the speaker are restricted and the optimization of high-power performances of the speaker working under low-frequency conditions is affected.
Another solution for detecting the vibration displacement is to provide a fixed pole plate and a movable pole plate on the speaker unit, and the movable pole plate is located on the vibration diaphragm. In this way, the vibration displacement can be monitored by detecting the capacitance value of a variable capacitor formed by the fixed pole plate and the movable pole plate. Although the solution can improve the monitoring accuracy of the vibration displacement, a connecting structure capable of connecting the two pole plates to an external circuit needs to be added, for example, the lead arrangement of various forms, etc. This not only increases the structural complexity and the difficulty of processing and assembling processes of the speaker module, but also increases potential fault points.